The Sims: Unleashed
Turtles originally appear in The Sims: Unleashed as pets. They can be bought from Old Town, provided there is already a turtle aquarium at home. Turtles do not have many interactions, but can die of starvation if they are not fed.
The Sims 3: Pets
Turtles appear again in The Sims 3: Pets in a more expanded form than in The Sims: Unleashed, with many breeds to choose from. They can be found in a sims backyard or on a random community lot or house. A sim can catch it, where it will be kept in their inventory until they are placed into an enclosure. They can die of starvation if not fed and can also be set free back into the wild.
The Sims 4: Island Living
Turtles make an appearance in The Sims 4: Island Living but as a background character instead of a pet. They are seen frequently swimming in the ocean in Sulani. They cannot be interacted with in any way and are just a part of the ocean. In the Mua Pel'am district in Sulani, occasionally a turtle hatching event will occur on the beach. Sims all around will flock to the hatching site and celebrate. The baby turtles however cannot be interacted with in any way, just like the adult turtles.